Paper is made from a stock contained over 99 percent water and less that 1 percent fiber by weight. The stock is formed into a web which has a water content of less than 5 percent by weight. The paper making process consists of forming a web from the dilute stock suspension of fibers in water by removing the water from the web. The removal of water from the formed web must typically be accomplished in a way which maximizes web strength, maximizes web thickness and minimizes cost.
Water is removed through three principal mechanisms: draining, pressing, and drying. Typically the draining process is the least costly. In draining, the fiber content of the web is taken from the stock fiber content of less than one percent to between about 10 and 15 percent. Next the fiber content is increased to between 30 and 35 percent fiber by pressing water from the web. The remainder of the water which must be removed to increase the web fiber content to 95 percent fiber is accomplished by drying the web.
Drying is typically accomplished by wrapping the web about steam heated dryer rolls and evaporating the water from the web. Drying is an expensive process because of the heat energy which must be supplied to evaporate water from the web. Because the drying process is relatively slow, the number and size of dryers required to remove the water remaining after pressing contributes to a considerable fraction of the overall cost of a papermaking machine. Furthermore, the dryers take up a majority of the volume occupied by the papermaking machine.
To improve the performance of papermaking machines and to facilitate operating at higher speed, the extended nip press was developed. Pressing a paper web typically involves passing the web supported between two press fabrics through a nip formed between two rolls. The pressure developed in the nip presses water from the web into the supporting press fabrics. The nip between two rolls has a width in the machine direction of less than one inch. The width of a nip between two rolls can be increased to a limited extent by making one or both of the rolls compliant. On the other hand an extended nip press employs a shoe which has a concave contact surface which engages the backing roll to form a nip having a width of 8 to 12 inches in the machine direction.
The extended nip press shoe is placed within a looped blanket which slides over the concave surface of the shoe on a film of oil. A paper web supported between pressing felts is passed between the backing roll and the blanket supported by the shoe. The shoe is mounted on a bearing pin to a hydraulic actuator which urges the shoe against the backing roll. The greater width of the nip in an extended nip press allows more time for water to move from the web into the press felt. By removing more water in the pressing section the web fiber content can be increased to forty to forty-five percent or more. This reduces the number of dryers required in the dryer section and the amount of energy required to dry the web. This results in a considerable increase in overall economic efficiency in a papermaking machine.
In an extended nip press, where the shoe is supported on a single pin, the pressure on the web as it moves through the extended nip is controlled by the shape of the shoe and the position of the pin beneath the shoe.
One problem with an extended nip press is that as the pressure in the nip decreases as the web leaves the nip, water can move from the press fabric back into the web, rewetting it. To overcome this problem, the pressure profile of the nip in the machine direction is tailored to increase the pressure towards the trailing or exit side of the nip. This increased pressure can overcome or decrease rewetting.
Various ways of achieving a pressure profile of a certain characteristic have been developed. The placement of the support pin and the shape of the shoe can control the pressure profile. Other techniques employ two hydraulic support actuators under the shoe. These techniques involve moving the center of support beneath the shoe. It is often desirable to be able to adjust the loading between the shoe and the backing roll as well as the shape of the pressure profile while the machine is running. Various furnishes used to form a paper web may require varying the total nip pressure and nip pressure profile.
Controlling paper quality over time may also require adjusting the total nip pressure as well as the shape of the nip pressure profile. Current techniques for performing this operation result in the center of support moving when the shape of the nip pressure profile is varied. Varying the center of support can limit the ability to independently control the two variables of nip pressure profile shape and nip total load.
What is needed is an extended nip press where the shoe support system can vary the nip loading and the shape of the pressure profile independently.